Subsurface drilling uses a drill string made up of a series of sections that are connected to one another end-to-end. The sections that couple together longitudinally to make a drill string may be called various names including “drill string sections”, “joints”, “tubulars”, “drill pipes”, or “drill collars”. Most commonly, the sections each have a pin end and a box end with complementary threads that are screwed together. The threads are commonly API standard threads.
When a well is being drilled, a drill bit is provided at the downhole end of the drill string. The drill bit drills a borehole that is somewhat larger in diameter than the drill string such that there is an annulus surrounding the drill string in the borehole. As the well is drilled, drilling fluid is pumped down through the drill string to the drill bit where it exits and returns to the surface through the annulus. The drilling fluid serves to counteract downhole pressures and keep the wellbore open. The drilling fluid also carries rock and other cuttings to the surface. As drilling progresses and the well bore gets deeper, new drill string segments are added at the uphole end of the drill string.
The first (farthest downhole) sections of a drill string are typically made up of heavy drill collars which, through their weight, apply pressure to the drill bit. This part of the drill string is typically called a bottom hole assembly or “BHA”. Above the BHA, the drill string sections may be lighter in weight.
Drilling is typically done using a drill rig. The drill rig includes equipment for rotating the drill string. In some cases, this equipment comprises a rotary table. In other cases, this equipment comprises a top drive. In either case, at the drill rig, as drilling progresses, new sections are added to the top of the drill string. This is done using equipment on the drill rig. Adding a new section typically involves supporting the drill string, uncoupling the top end of the drill string from the kelly or top drive that was supporting it, coupling a new section to the top end of the drill string, connecting the uphole end of the new section to the kelly or top drive and resuming drilling. Typically the weight of the drill string is carried by slips on the drill rig floor while a new section is being added to the drill string.
Periodically, as drilling progresses, it is usually necessary to retrieve the drill string from the well bore. This may be required, for example, to replace the drill bit if the drill bit is becoming worn. This also may be required in cases where there is downhole equipment of some kind that needs to be retrieved to the surface for servicing. The process of bringing a drill string back to the surface and returning the drill string into a partially completed well bore is called “tripping”. Since well bores may be many thousands of feet deep, tripping may take a long time to complete. Operating a drill rig is very expensive. Consequently, tripping can contribute large costs.
Some systems have been proposed for making tripping a drill string more efficient. These include U.S. Pat. No. 8,844,616 and US20140124218. These systems have various disadvantages. Practical alternatives to these systems are required.
There is a general need for ways to improve the efficiency of subsurface drilling. There is a particular need to reduce the time taken to trip a drill string.